Troubleshooting PVST Issues: A Comprehensive Guide

PVST (Per-VLAN Spanning Tree) is a Cisco proprietary protocol used for loop avoidance in VLANs.

While it is a robust technology, issues may arise that require troubleshooting.

Let’s discuss the common PVST issues, their causes, and how to troubleshoot them.

Common PVST Issues and Their Causes

In this section, we will discuss the most common issues that may occur in a PVST network and their root causes.

Blocked Ports

One of the most common PVST issues is blocked ports. A blocked port is a port that is not forwarding traffic, but it is not completely disabled either. A blocked port is necessary for loop prevention, but it can cause issues if it is not configured properly.

Causes of Blocked Ports

Blocked ports can be caused by incorrect STP configuration, such as incorrect port priorities, or by physical issues, such as a damaged cable or switch port.

How to Troubleshoot Blocked Ports

To troubleshoot blocked ports, you should first check the STP configuration to make sure that the port priority is set correctly. You should also check for physical issues, such as a damaged cable or switch port.

Root Bridge Issues

Another common PVST issue is root bridge issues. The root bridge is the central point of the STP topology and is responsible for forwarding traffic to all other switches in the network. If there are issues with the root bridge, the network may experience connectivity issues.

Causes of Root Bridge Issues

Root bridge issues can be caused by incorrect STP configuration, such as incorrect root bridge selection, or by physical issues, such as a damaged switch or cable.

How to Troubleshoot Root Bridge Issues

To troubleshoot root bridge issues, you should first check the STP configuration to make sure that the root bridge is selected correctly. You should also check for physical issues, such as a damaged switch or cable.

Inconsistent VLAN Information

Inconsistent VLAN information is another common PVST issue. Inconsistent VLAN information occurs when different switches have different VLAN information, which can cause connectivity issues.

Causes of Inconsistent VLAN Information

Inconsistent VLAN information can be caused by incorrect VLAN configuration, such as incorrect VLAN ranges, or by physical issues, such as a damaged cable or switch port.

How to Troubleshoot Inconsistent VLAN Information

To troubleshoot inconsistent VLAN information, you should first check the VLAN configuration to make sure that the VLAN ranges are set correctly. You should also check for physical issues, such as a damaged cable or switch port.

Inconsistent Bridge Priority

Inconsistent bridge priority is another common PVST issue. Inconsistent bridge priority occurs when different switches have different bridge priorities, which can cause connectivity issues.

Causes of Inconsistent Bridge Priority

Inconsistent bridge priority can be caused by incorrect STP configuration, such as incorrect bridge priorities, or by physical issues, such as a damaged switch or cable.

How to Troubleshoot Inconsistent Bridge Priority

To troubleshoot inconsistent bridge priority, you should first check the STP configuration to make sure that the bridge priorities are set correctly. You should also check for physical issues, such as a damaged switch or cable.

VLAN Mismatch

VLAN mismatch is another common PVST issue. VLAN mismatch occurs when different switches have different VLAN information, which can cause connectivity issues.

Causes of VLAN Mismatch

VLAN mismatch can be caused by incorrect VLAN configuration, such as incorrect VLAN ranges, or by physical issues, such as a damaged cable or switch port.

How to Troubleshoot VLAN Mismatch

To troubleshoot VLAN mismatch, you should first check the VLAN configuration to make sure that the VLAN ranges are set correctly.

You should also check for physical issues, such as a damaged cable or switch port.

Troubleshooting PVST Issues

In this section, we will provide step-by-step guidelines to troubleshoot PVST issues. We will also discuss some useful commands to verify PVST operation.

Checking PVST Configuration

The first step in troubleshooting PVST issues is to check the PVST configuration on all switches in the network. This includes checking the STP mode (PVST or PVST+), the root bridge selection, the bridge priority, and the VLAN ranges.

To check the PVST configuration, use the following command:

show spanning-tree

This command displays the current STP configuration, including the root bridge, the bridge priority, and the VLAN ranges.

Checking VLAN Information

The next step is to check the VLAN information on all switches in the network. This includes checking the VLAN ranges and making sure that all switches have consistent VLAN information.

To check the VLAN information, use the following command:

show vlan

This command displays the current VLAN configuration, including the VLAN ranges and the ports assigned to each VLAN.

Identifying Root Bridge Issues

If there are issues with the root bridge, the network may experience connectivity issues. To identify root bridge issues, use the following command:

show spanning-tree root

This command displays information about the root bridge, including the root bridge ID and the port used to connect to the root bridge.

Identifying Blocked Ports

Blocked ports can cause connectivity issues in the network. To identify blocked ports, use the following command:

show spanning-tree blockedports

This command displays information about blocked ports, including the port ID and the reason for the port being blocked.

Identifying Inconsistent Bridge Priority

Inconsistent bridge priority can cause connectivity issues in the network. To identify inconsistent bridge priority, use the following command:

show spanning-tree inconsistentports

This command displays information about inconsistent bridge priority, including the port ID and the reason for the inconsistency.

Resolving VLAN Mismatch Issues

If there are VLAN mismatches in the network, connectivity issues may occur.

To resolve VLAN mismatch issues, you should make sure that all switches have the same VLAN information.

This can be done by checking the VLAN configuration on all switches and making any necessary changes.

Useful Commands to Verify PVST Operation

In addition to the commands mentioned above, there are several other useful commands to verify PVST operation. These include:

• show interfaces: Displays information about interfaces on a switch, including the interface status and VLAN information.
• show spanning-tree summary: Displays a summary of the STP configuration, including the number of blocked ports and the root bridge ID.
• show spanning-tree detail: Displays detailed information about the STP configuration, including the bridge ID, the root bridge ID, and the port roles.

By using these commands, you can quickly identify any issues with PVST operation and take the necessary steps to resolve them.

Conclusion

In conclusion, PVST is a crucial protocol for ensuring loop avoidance in VLANs. However, issues may arise that require troubleshooting, which can be complex and time-consuming.

By following the best practices for PVST configuration and regularly checking the network for any issues, network administrators can prevent issues from occurring in the first place.

If you want to become an expert in PVST and other advanced networking technologies, consider taking Orhan Ergun's CCNP ENCOR 350-401 course.

With this course, you'll learn advanced concepts in networking, including automation, security, and troubleshooting. You'll also gain practical experience with hands-on labs and real-world scenarios.